1. Field of the Invention
The present invention relates to a lateral electric field liquid crystal display device which implements an active matrix type liquid crystal display device and the like excellent in the viewing angle characteristics.
2. Description of the Related Art
The widely used TN (Twisted Nematic) mode is of high contrast. On the other hand, the molecule axes of the liquid crystal of that mode rise by the vertical electric field, so that the viewing angle dependency is tremendous. Recently, it is desired to acquire the same picture quality when viewed from any directions regarding display device for large-scaled monitors such as TVs and regarding portable information terminals. In order to satisfy such demands, employed more and more are the modes with which the liquid crystal is rotated on a plane substantially in parallel to the substrate by applying an electric field substantially parallel to the substrate, such as an IPS (In-plane Switching) mode and an FFS (Fringe Field Switching) mode. With such lateral electric field modes, the axes of molecules of the nematic liquid crystal aligned horizontally are rotated within a plane that is in parallel to the substrate by the lateral electric field. This makes it possible to suppress changes in the picture quality caused by the viewing angle directions according to the rise of the axes of the molecules, so that the viewing angle characteristics can be improved.
However, the viewing angle characteristics are not perfect even in the case of the lateral electric field mode. In particular, the nematic liquid crystal used for the lateral electric field mode exhibits the uniaxial optical anisotropy. Thus, there is acquired prescribed retardation when viewed from the normal direction of the substrate. However, as shown in FIG. 16, when viewed from an oblique viewing direction by tilting the viewing angle from the normal of the substrate, there are different changes in the retardation caused by the liquid crystal layer for the case where the viewing angle is tilted to the major-axis direction of the liquid crystal and for the case where the viewing angle is tilted to the minor-axis direction of the liquid crystal. In the case where the viewing angle is tilted to the minor-axis direction, the refractive index anisotropy of the liquid crystal on the appearance does not change. Thus, the optical path length transmitting the liquid crystal layer becomes greater, so that the retardation Δn·d becomes greater. Meanwhile, in the case where the viewing angle is tilted to the major-axis direction, the refractive index anisotropy of the liquid crystal on the appearance becomes smaller. Thus, the retardation Δn·d becomes smaller, even though the optical path length transmitting the liquid crystal layer becomes longer. Normally, with the lateral electric field mode, black display is acquired by making the alignment direction of the liquid crystal aligned with one of absorption axes 28 and 29 (FIG. 16) of the crossed Nichol polarization plate by applying no voltage and white display is acquired through rotating the liquid crystal from the polarization axis direction by applying a lateral electric field. In that state, the effective retardation becomes smaller when viewed from the oblique viewing angle of the azimuth of the rotated liquid crystal due to the reasons described above, so that the chromaticity is shifted to the direction of blue. When viewed from the oblique direction perpendicular to the azimuth of the rotated liquid crystal, the effective retardation becomes greater. Thus, the chromaticity is shifted to the direction of red. Therefore, both cases are to be colored.
Further, as shown in FIG. 17, polarization axes 60 and 61 are orthogonal to each other from the front direction, and the liquid crystal rotates therebetween to control the transmission light. However, when viewed from the oblique viewing angle direction at the azimuth of 45 degrees from the polarization axis, the transmission axes of the polarization plate do not become orthogonal to each other as shown in FIG. 17B and FIG. 17C, so that an azimuth 62 of ordinary light axis of the liquid crystal comes to rotate between the non-orthogonal polarization axes. Therefore, in the oblique viewing direction in a state where the azimuth 62 of the ordinary light axis of the liquid crystal is facing towards the polarizer absorption axis (black display from the front), light is leaked so that the black display state becomes brighter. Further, when viewed from the viewing direction in a layout as shown in FIG. 17B, the luminance is decreased at the point where the liquid crystal is slightly rotated from the black display state. This results in generating inversion of gradation.
In the technique depicted in JP No. 3120751 (Patent Document 1), as shown in FIG. 20A, disclosed is a method with which the directions of an electric field 70 applied to the liquid crystal are set to two mutually opposite directions which make a specific angle with respect to an initial alignment direction 69 of the liquid crystal. Through setting the electric field 70 to be applied from the two directions as described above, the liquid crystal rotates in different directions from each other in a region 1 and a region 2 provided that each of the regions where the electric fields are generated is defined as the region 1 (65) and the region 2 (66).
When viewed from the oblique viewing angle at an azimuth 71 of the viewing angle making 45 degrees with respect to the absorption axes 28 and 29 of the both polarization plates, the liquid crystal is to be rotated to the two directions making at about 45 degrees from the direction of the polarization axes for white display. Thus, as shown in FIG. 20B, the major-axis direction and the minor-axis direction from the oblique direction of the liquid crystal in the both regions compensate with each other. Therefore, it is possible to suppress coloring observed from the oblique directions as described in FIG. 16.
Further, as shown in FIG. 20C, among four quadrants formed by the non-orthogonal polarization axes 60 and 61, liquid crystal directors in the region 1 are rotated in a quadrant where the angle formed by the polarization angles is an obtuse angle while the liquid crystal directors in the region 2 are rotated in a quadrant where the angle formed by the polarization angles is an acute angle. Thus, the both regions compensate with each other, so that the inversion of gradation viewed from the oblique direction at 45 degrees can also be suppressed.
The technique of Patent Document 1 described above is designed to rotate the liquid crystal by applying the voltage between two kinds of strip electrodes 63 and 64 by the lateral electric field 70 generated therebetween. In the meantime, recently, widely used is the so-called FFS-mode lateral electric field liquid crystal display device in which, as shown in FIG. 28A and FIG. 28B, a plan electrode 82 is formed on a substrate 81, a strip electrode 84 is disposed thereon via an insulating film 83, a voltage is applied therebetween, and a fringe electric field substantially in parallel to the substrate 81 generated at the edges of the strip electrode 84 is used to rotate a liquid crystal 85.
Through the use of such lateral electric field liquid crystal display device that utilizes the fringe electric field, the liquid crystal on the electrodes can also be rotated. Therefore, the light use efficiency can be increased even more. Further, with the FFS mode, the rotation of the liquid crystal becomes dominant on the substrate side where the fringe electric field is formed. Thus, compared to the rotation of the liquid crystal by the pure lateral electric field, the dependency of the electro-optical property on the thickness of the liquid crystal layer becomes smaller and the margin of the liquid crystal cell gap becomes greater. Therefore, the difficulties of manufacturing can be eased.
However, in the case of the FFS mode, the voltage-transmittance characteristics is largely shifted towards the low-voltage side as shown in FIG. 29 when the viewing angle is tilted towards the initial alignment direction of the liquid crystal in particular. Thus, the delicate coloring using the half gray tone level becomes whiter in the oblique viewing angle direction.
As a result of analyzing such phenomenon, it is found that there are two following reasons. As shown in FIG. 18A, a case where the viewing angle is tilted by η from the normal of the substrate towards the azimuth of the polarization axis of the incident-side polarization plate, is considered. The unit vectors in the absorption axis direction of the orthogonal polarization plates when viewed from the front are defined as the p for the polarizer and a for the analyzer. Considering the state where the liquid crystal director is rotated by θ from the initial state, the director n of the liquid crystal can be expressed as follows.n=cos θ·p+sin θ·a 
Provided that the unit vector in the direction of a light propagation is s and that the transmission axis directions of the polarization plates perpendicular to the light propagation are p′, a′ and the axis direction of the ordinary light of the liquid crystal orthogonal to the light propagation is n′, following relations can be acquired.p′=p×s a′=a×s n′=n×s=cos θ·p′+sin θ·a′
While p′ and a′ are orthogonal to each other, the lengths thereof are different as shown in the following expressions.|p′|=cos η|a′|=1
Therefore, as shown in FIG. 18B, the angle φ formed between n′ and p′ becomes larger than θ.
In this case, the transmission can be acquired by a following expression.T∝ cos2(π/2−2φ)=sin2(2φ)>sin2(2θ)
Therefore, the transmittance of the oblique viewing angle becomes relatively larger in the region where θ is small compared to “θ−T” characteristics from the front when θ changes. Thus, the peak is at the point where φ corresponds to 45 degrees. When φ becomes equal to or larger than that, the transmittance is decreased inversely and deviated from the ideal characteristics.
In the case of FFS, the rotation angle becomes larger because a strong lateral electric field is generated in the vicinity of the edges of the strip electrodes while the electric field is weak and the rotation angle is small on the strip electrodes as well as in the part corresponding to the slits between the strip electrodes. Thus, a high transmittance can be acquired by rotating the liquid crystal on the average in those regions. Therefore, in the vicinity of the edges of the strip electrodes with high light use efficiency, the liquid crystal is largely rotated from the region where the voltage is relatively low. Thus, the rotation angle φ when viewed from the oblique viewing angle becomes still larger. As a result, the rotation angle φ of the liquid crystal when viewed from the oblique viewing angle exceeds 45 degrees with a voltage that is considerably lower than the voltage with which the highest transmittance can be acquired from the front view, which eminently causes a phenomenon of transmittance saturation.
In a case where the lateral electric field 70 is applied between the two kinds of strip electrodes 63 and 64 as shown in FIG. 20A, the liquid crystal is mainly rotated by the lateral electric field 70 generated between the electrodes 63 and 64. Thus, the liquid crystal is not rotated so much on the strip electrodes 63, 64 and transmittance nearby is low. However, it is not necessary to increase the rotation of the liquid crystal between the electrodes 63 and 64 so much. Therefore, while the shift as described above is generated slightly, the level of the shift is so small that it is not an issue.
As shown in FIG. 21A, considered is a case where there are two directions of the electric field 70 for the initial alignment 69 of the liquid crystal in a case of the FFS mode. In this case, it is possible to improve the viewing angle regarding the coloring and the inversion of gradation in a case of viewing from the azimuth of 45 degrees from the polarization plates due to the same reason for which the electric field is applied from two directions in the mode where the lateral electric field is applied between the strip electrodes shown in FIG. 20A.
However, as shown in FIG. 21A, it is not possible to suppress the low-voltage shift of the voltage-transmittance characteristics when viewed from the oblique direction of the azimuth of p. This can be described as follows. The ordinary light directions n1′ and n2′ of the liquid crystal perpendicular to the light propagation lay can be expressed as in following expressions.n1′=n1×s=cos θ·p′+sin θ·a′n2′=n2×s=cos θ·p′−sin θ·a′
As shown in FIG. 21B, the angle φ formed between n′ and p′ is equivalent in the both regions. Thus, the liquid crystal is rotated faster than the case of the rotation angle θ of the front view, so that it is not possible to compensate with each other even when the electric field 70 is applied to two directions. Therefore, it is not possible to overcome such issue that the voltage-transmittance characteristics is shifted to the low-voltage side, and the display thereby appears whiter in a bright halftone and that a delicate coloration cannot be displayed correctly in the oblique viewing angle.
Further, liquid crystal molecules generally have pretilt. When an electric field is applied, the liquid crystal molecules tend to rise in the rising direction of the pretilt. When the liquid crystal rises as in this case, the ordinary light direction n′ of the liquid crystal viewed from the oblique viewing angle shifts to the direction of z′ given by a following expression provided that z is the unit vector of the direction perpendicular to the substrate as shown in FIG. 19.z′=z×s 
Therefore, the rotation angle θ of the liquid crystal becomes still larger, so that the shift of the voltage-transmittance characteristics towards the low-voltage direction becomes still greater when viewed from the oblique viewing angle of the direction of the rise of the pretilt. As a result, white-tinged display with a light halftone becomes dominant.
In view of the above-described factors, it is an exemplary object of the present invention to provide a fine display device with which the delicate coloration of a halftone does not appear as white-tinged when viewed from any viewing angles by suppressing the shift of the voltage-transmittance characteristics to the low-voltage side when viewed from the oblique viewing angle of the azimuth of the initial alignment of the liquid crystal in an FFS mode that is capable of more easily increasing the transmittance.